1. Field of the Invention
The present invention relates to a mobile communications system, and more particularly, to a method and apparatus for controlling a data transmission rate for a reverse link, in which a traffic-to-pilot power ratio is used to set the data transmission rate of a terminal under handoff.
2. Discussion of the Related Art
In the world of cellular telecommunications, those skilled in the art often use the terms 1G, 2G and 3G. The terms refer to the generation of the cellular technology used. 1G refers to the first generation, 2G to the second generation, and 3G to the third generation.
1G is used to refer to the analog phone system, known as an AMPS (Advanced Mobile Phone Service) phone systems. 2G is commonly used to refer to the digital cellular systems that are prevalent throughout the world, and include CDMAOne, Global System for Mobile communications (GSM), and Time Division Multiple Access (TDMA). 2G systems can support a greater number of users in a dense area than can 1G systems.
3G is commonly used to refer to the digital cellular systems currently being developed. Recently, third-generation (3G) CDMA communication systems have been proposed including proposals, such as cdma2000 and W-CDMA. These 3G communication systems are conceptually similar to each other with some significant differences.
A cdma2000 system is a third-generation (3G) wideband; spread spectrum radio interface system which uses the enhanced service potential of CDMA technology to facilitate data capabilities, such as Internet and intranet access, multimedia applications, high-speed business transactions, and telemetry. The focus of cdma2000, as is that of other third-generation systems, is on network economy and radio transmission design to overcome the limitations of a finite amount of radio spectrum availability.
Referring to FIG. 1, a wireless communication network architectures is illustrated. A subscriber uses a Mobile Station 2 to access network services. The Mobile Station 2 may be a portable communications unit, such as a hand-held cellular phone, a communication unit installed in a vehicle, or a fixed-location communications unit.
The electromagnetic waves from the Mobile Station 2 are transmitted by the Base Transceiver System (BTS) 3 also known as node B. The BTS 3 consists of radio devices such as antennas and equipment for transmitting radio waves. The Base Station Controller (BSC) 4 receives the transmissions from one or more BTS's. The BSC 4 provides control and management of the radio transmissions from each BTS 3 by exchanging messages with the BTS and the Mobile Switching Center (MSC) 5 or Internal IP Network. The BTS's 3 and BSC 4 are part of the Base Station (BS) 6.
The BS 6 exchanges messages with and transmits data to a Circuit Switched Core Network (CSCN) 7 and Packet Switched Core Network (PSCN) 8. The CSCN 7 provides traditional voice communications and the PSCN 8 provides Internet applications and multimedia services.
The Mobile Switching Center (MSC) 5 portion of the CSCN 7 provides switching for traditional voice communications to and from a Mobile Station 2 and may store information to support these capabilities. The MSC 2 may be connected to one of more BS's 6 as well as other public networks, for example a Public Switched Telephone Network (PSTN) (not shown) or Integrated Services Digital Network (ISDN) (not shown). A Visitor Location Register (VLR) 9 is used to retrieve information for handling voice communications to or from a visiting subscriber. The VLR 9 may be within the MSC 5 and may serve more than one MSC.
A user identity is assigned to the Home Location Register (HLR) 10 of the CSCN 7 for record purposes such as subscriber information, for example Electronic Serial Number (ESN), Mobile Directory Number (MDR), Profile Information, Current Location, and Authentication Period. The Authentication Center (AC) 11 manages authentication information related to the Mobile Station. The AC 11 may be within the HLR 10 and may serve more than one HLR. The interface between the MSC 5 and the HLR/AC 10, 11 is an IS-41 standard interface 18.
The Packet Data Serving Node (PDSN) 12 portion of the PSCN 8 provides routing for packet data traffic to and from Mobile Station. The PDSN 12 establishes, maintains, and terminates link layer sessions to the Mobile Station's 2 and may interface with one of more BS 6 and one of more PSCN 8.
The Authentication, Authorization and Accounting (AAA) 13 Server provides Internet Protocol authentication, authorization and accounting functions related to packet data traffic. The Home Agent (HA) 14 provides authentication of MS 2 IP registrations, redirects packet data to and from the Foreign Agent (FA) 15 component of the PDSN 8, and receives provisioning information for users from the AAA 13. The HA 14 may also establish, maintain, and terminate secure communications to the PDSN 12 and assign a dynamic IP address. The PDSN 12 communicates with the AAA 13, HA 14 and the Internet 16 via an Internal IP Network.
FIG. 2 illustrates a data link protocol architecture layer 20 for a wireless network. It includes an upper layer 60, a link layer 30 and a physical layer 21.
The upper layer 60 contains three basis services; voice services 62, data services 61 and signaling 70. Voice services 62 include PSTN access, mobile-to-mobile voice services, and Internet telephony. Data services 61 are services that deliver any form of data on behalf of a mobile end user and include packet data applications such as IP service, circuit data applications such as asynchronous fax and B-ISDN emulation services, and SMS. Signaling 70 controls all aspects of mobile operation.
The Link Layer 30 is subdivided into the Link Access Control (LAC) sublayer 32 and the Medium Access Control (MAC) sublayer 31. The link layer 30 provides protocol support and control mechanisms for data transport services and performs the functions necessary to map the data transport needs of the upper layer 60 into specific capabilities and characteristics of the physical layer 21. The Link Layer 30 may be viewed as an interface between the upper layers and the Physical Layer 20.
The separation of MAC 31 and LAC 32 sublayers is motivated by the need to support a wide range of upper layer 60 services, and the requirement to provide for high efficiency and low latency data services over a wide performance range (from 1.2 Kbps to greater than 2 Mbps). Other motivators are the need for supporting high QoS delivery of circuit and packet data services, such as limitations on acceptable delays and/or data BER (bit error rate), and the growing demand for advanced multimedia services each service having a different QoS requirements.
The LAC sublayer 32 is required to provide a reliable, in-sequence delivery transmission control function over a point-to-point radio transmission link 42. The LAC sublayer 32 manages point-to point communication channels between upper layer 60 entities and provides framework to support a wide range of different end-to-end reliable link layer 30 protocols.
The MAC sublayer 31 facilitates complex multimedia, multi-services capabilities of 3G wireless systems with Quality of Service (QoS) management capabilities for each active service. The MAC sublayer 31 provides procedures for controlling the access of data services (packet and circuit) to the physical layer 21, including the contention control between multiple services from a single user, as well as between competing users in the wireless system. The MAC sublayer 31 also provides for reasonably reliable transmission over the radio link layer using a Radio Link Protocol (RLP) 33 for a best-effort level of reliability. Signaling Radio Burst Protocol (SRBP) 35 is an entity that provides connectionless protocol for signaling messages. Multiplexing and Quality of Service (QoS) Control 34 is responsible for enforcement of negotiated QoS levels by mediating conflicting requests from competing services and the appropriate prioritization of access requests.
The Physical Layer 20 is responsible for coding and modulation of data transmitted over the air. The Physical Layer 20 conditions digital data from the higher layers so that the data may be transmitted over a mobile radio channel reliably.
The Physical Layer 20 maps user data and signaling, which the MAC sublayer 31 delivers over multiple transport channels, into a physical channels and transmits the information over the radio interface. In the transmit direction, the functions performed by the Physical Layer 20 include channel coding, interleaving, scrambling, spreading and modulation. In the receive direction, the functions are reversed in order to recover the transmitted data at the receiver.
An optimum rate of data transmission in the reverse link of a mobile communications system, for example, a first-evolution data-optimized (1xEV-DO) system, is determined with respect to the rise-over-thermal of a given base station 6. The rise-over-thermal is a dynamic reception characteristic defined as total power of signal of total power received at the base station 6, from all active mobile stations 2 (also referred to as terminals), and the thermal noise detected at the base station. In other words, the rise-over-thermal is the summed signal power of all active-terminal signals received at the base station 6, which is a function of reverse activity, i.e., the number and transmission rate of active terminals 2 operating in connection with the base station.
Ideal reverse-link conditions result when the rise-over-thermal at the base station 6 is maintained at a constant level despite fluctuations in reverse activity, such that the rise-over-thermal is a function of the various transmission rates for a given number of active terminals 2. Thus, the system compensates by controlling inter alia the data transmission rate of the reverse link of each terminal 2.
To enable such control, the rise-over-thermal is compared with a threshold value, and based on the comparison results, an active terminal 2 is requested to increase or decrease its transmission rate when communicating with the base station 6. That is, the transmission rate may be increased when the rise-over-thermal is below the threshold, but if the rise-over-thermal exceeds the threshold, it is necessary to decrease the transmission rate.
FIG. 3 illustrates a reverse-link transmission rate control method 50 according to a related art. As illustrated in FIG. 3, one base station 6 and one active terminal 2 of a 1xEV-DO system act together in each frame to set an optimum rate for the next frame of the reverse link.
In step S52, the base station 6 measures the rise-over-thermal (RoT) produced by the cumulative effect of all reverse-link data signals power. With the rise-over-thermal thus determined, the base station 6 generates, in step S54, a reverse activity bit (RAB) as part of an instruction word for use by a terminal 2. As described above, the RAB value or parameter is set according to a comparison of the rise-over-thermal and a predetermined threshold value, whereby one value would instruct the terminal 2 to decrease its transmission rate, and another value would instruct the terminal to increase its transmission rate.
In step S56, the base station 6 transmits the reverse activity bit to all active terminals 2 within active sectors, or all terminals transmitting data on the reverse link via a random access channel, which is a common channel. Thus, all terminals 2 simultaneously receive an instruction word containing the same reverse activity bit for a given frame, such that all terminals are simultaneously instructed to increase or decrease their set rate of data transmission for the next frame.
In step S58, a terminal 2 receiving the reverse activity bit performs a compliance test to determine whether the data transmission rate should be changed based on the received bit. The terminal 2 considers the data rate of the current frame of the reverse link transmission and, using a predetermined algorithm, determines either to comply with the instruction from the base station 6 and change the transmission rate accordingly or to ignore the instruction and set the transmission rate of the next frame equal to that of the current frame. In step S60, the terminal 2 sets the data transmission rate of the next frame.
In the aforementioned method 50 according to the related art, the reverse activity bit is generated based solely on the rise-over-thermal measured at the base station 6 and the bit is simultaneously transmitted as a single command to all active terminals 2 within active sectors. In other words, there is no consideration of the status of any one of the terminals 2. There are inherent disadvantages in this method.
For any given terminal 2, the only option other than complying with the instruction from the base station 6 is to ignore the instruction and maintain the current data transmission rate. Therefore, since the terminal 2 cannot consider its current status in determining whether to change its transmission rate, reverse-link transmission efficiency tends to suffer.
On the other hand, any given terminal 2 receiving a reverse activity bit may comply with the corresponding instruction or ignore the instruction based on the results of its own compliance test, and, therefore, may not change its transmission rate. Therefore, effective regulation of the rise-over-thermal by a base station 6 is hindered, which also degrades reverse-link transmission efficiency.